The present invention relates generally to a slip control apparatus for use in motor vehicles, and more particularly to a control system for preventing excessive slip from occurring during starting and acceleration of the motor vehicle.
One example of such a control system is shown in Japanese Patent Provisional Publication No. 61-108040 and is so constructed that the rotational speed and rotational acceleration of a driven wheel of.the motor vehicle are respectively compared with upper limits and lower limits for detection of an acceleration slip of the driven wheel during starting and/or acceleration of the motor vehicle so that driven-wheel acceleration slip is prevented by independently controlling a throttle valve of the internal combustion engine and a braking device of an anti-skid control system employed for the motor vehicle.
Generally, for quickly suppressing the initial wheel acceleration slip, the braking control tends to be more effective as compared with the throttle valve control because of the response time lag of the internal combustion engine and the associated drive-force transmission system. Thus, the simultaneous independent control of the braking device and the throttle valve may result in generation of an excessive engine torque due to the delay of the throttle valve control with respect to the braking control. In addition, the prior system starts the slip prevention control only after the detection of occurrance of the acceleration slip and therefore a further problem arises with such a prior system in that difficulty is encountered in preventing the wheel from racing at the initial time of starting and/or acceleration of the motor vehicle. This can decrease the road surface friction coefficient to make starting the motor vehicle impossible. Furthermore, a further important problem in such a prior system relates to malfunction of the system on turning of the vehicle. That is, during turning, the wheels are driven at different speeds from each other and the driven wheel can rotate at a higher speed as compared with the non-driven wheel. In this case, there is the possibility that the system falls into an error in terms of detection of the occurrance of the acceleration slip.